Seismic surveying typically involves the use of a source of seismic energy and an array of seismic detectors strategically positioned for its reception. The source of seismic energy can be an apparatus capable of delivering a series of impacts or mechanical vibrations to the surface of the earth, the detonation of a high explosive charge near the earth surface, or other means capable of generating seismic wave energy. The resulting acoustic waves generated in the earth, including those which are reflected from earth strata interfaces, are detected by seismic detectors which transduce the acoustic waves into representative electrical signals. From these electrical signals, informational data may be deduced concerning the structure of earth's substrata.
In vertical seismic profiling, the seismic detectors are positioned at different depths in a borehole, such as a well bore, and the signals from the detectors in response to reception of energy from a seismic energy source are recorded and grouped in alignment in a single display in the order of detector depth. From this display, coherences between the signal traces may be noted which may be analyzed and interpreted to provide information regarding the geologic substructure. However, because of the multiplicity of components comprised in the seismic energy received by a detector and its representative electrical signal, analysis of the signals is oftentimes exceedingly difficult. Such components will typically include a downgoing component representing the directly arriving wave field propagated from the energy source, other downgoing components from seismic energy which have undergone multiple reflections in geologic strata above the detector, upgoing components from seismic energy reflected from interfaces of geologic strata or structures located below the detector, and spurious waves of various kinds.
Heretofore, a number of field-operating and data processing techniques have been devised to accentuate the upgoing components in the detector signals and at the same time minimize the interfering effects of the downgoing components since it is the upgoing components and their transit times representative of reflections from substrata interfaces which provide most useful information and are of primary concern. Most of the current techniques for separating the opposite directions of seismic wave travel in the detector signals assume that time shifts which align the first breaks in the signal traces can also be used to align the desired upgoing seismic wave events. However, this assumption is strictly true only where the upgoing events are derived from horizontal reflectors and the detector signals are from detectors located in a vertical borehole with zero source offset, i.e., the seismic source is located as close to the borehole as possible rather than being offset therefrom. In particular, the current techniques are of limited effectiveness in compensating for "dipping" reflectors and in offset vertical seismic profiling used to identify geologic substructures, such as faults or salt domes located a distance from the well bore. Also, the multichannel "dip" filters in current use generally require many more than two detector signals from which to extract desired information.
The new filtering process described herein does not require preprocessing to align seismic events in seismic signal traces and is applicable to a wide range of geometries, including vertical seismic profiling applications wherein the seismic source is offset from the well bore. It also makes possible, by means of two-trace filtering, to reduce the number of receiver depth levels required in the acquisition of a vertical seismic profile, and therefore a reduction in acquisition costs.